Diffusion vacuum pump



Dec.8, 1936. H. w.- EDWARDS 2,063,665,

- I DIFFUSION VACUUM PUMP Filed March 21, 1954 I 2 K Ill I T 41 31 I cil 50 ai 15 F gl Jr |'.l 21 .36 l H E 15 2.. (L r '17 l 1%)" fiiidirards,by:

' .fliiorneg/ Patented Dec. 8, 1936 UNITED STATES PATENT OFFlCEDIFFUSION VACUUM PUMP and L. H. McGowan Application March 21, 1934,Serial No. 716,639

8 Claims.

My invention relates to diffusion pumps of vapor type, which are adaptedfor obtaining high vacuum in closed receptacles, such as vacuum chambersused for condensing metal in vapor phase on mirror bodies and forproducing high vacua in any other closed receptacles desired. Among theobjects of my invention is the production of a high speed vacuum pumpwhich shall be as fast and efficient in operation as possible and whichshall be extremely simple in construction, and free from moving parts.One of the characteristics of pumps of this type heretofore in use ishigh resistance in the passage or conduit leading from the source ofvapor to the condenser, which has impaired the operation of the pump,increased the amount of heat required and slowed down the effectiveresults obtained. One object of this invention is to reduce thisresistance to a 20 minimum and permit the vapor to be given acorrespondingly high velocity between its source and the condenser.Another object of the invention is to cause practically all thecondensed liquid to return freely to the source of vapor. Another objectis the introduction of a nonmetallic vapor into the inethod of operatingmy improved pump, whereby the presence or even minute traces of vapor ofmetallic origin from the pump, in the vacuum chamber is prevented. 30This is an advantage when metallic alloys of fine proportions in vaporphase are being condensed in the vacuum chamber. A still further objector feature of my invention is the use of oil of low heat ofvaporization, in my improved pump, 35 whereby quicker and more eilectiveresults are attained.

In the accompanying drawing forming part of this specification, Fig. 1is a diagrammatic view, showing the application of my improved 40diffusion pump in use; Fig. 2 is a vertical, central section, of myimproved difiusion pump, taken on line 2-2 of Fig. l, and Fig. 3 is aperspective view of one of the baiiies.

The difiusion pump forming the subject of 45 this invention is designedparticularly, although not exclusively for use with a mineral oil whichhas a low vapor pressure and may therefore be used without a liquid airor solid carbon dioxide trap, which is necessary in most cases if low 50pressures are to be obtained. The particular pump herein described willproduce about one millionth of a millimeter of mercury pressure and hasa diameter of three inches. This diameter may be varied and whereverspecific dimensions are mentioned throughout this speci- 5 fication, Ido not wish to .be limited thereby, within the spirit of my invention.

My improved pump consists of a tubular casing A, made of metal or othersuitable material, whichis intended to be used in substantially uprightposition. The lower end of this casing is sealed with a circular plateforming a floor. H), of the same material as the tube and the upper endis intended to be connected in any suitable manner toa vacuum chamber Bor any other device from which air is to be exhausted. The lower closedend of the casing serves as a vaporizer and reservoir for a supply ofoil H or other suitable vaporizing material. I have found that by usingoil of low heat vaporization, quicker and better results are obtained inthe operation of my improved pump. A condenser, resembling a waterjacket C and using water or other cooling medium circulated throughingress and egress ducts l2 and I3 is located approximately in themedian region of the casing and adapted to condense the oil or othervapor. A-suitable heater of any kind desired is placed below and incontact with the lower end it of the pump in order to heat the oilsuificiently to supply a stream of oil vapor upwardly. Said heater isindicated diagrammatically by the electric resistance coil M in thereceptacle I5, the latter being preferably a poor heat conductor made ofasbestos or other suitable heat resistance material, within which thelower end of the tube A is enclosed, thus surrounding the regioncontaining the vaporizing oil.

The internal parts of the pump are all removable from its upper end tofacilitate cleaning when desired. These internal parts are a chimney D,a deflector E, and the baflie system F. The chimney is preferably madeout of any good heat conducting material such as copper. Its lower endfor a certain distance upwardly, about the height of the receptacle l5,conforms closely with the inner diameter of tube A and is removableupwardly therefrom. Above its lower endthe chimney contracts into a longcylindrical,

upwardly extending body bearing a definite ratio to the inside diameterof the pump which has been found to be suitable for a maximum speed at adefinite pressure. Approximately this ratio is as follows: when tube Ais fifteen inches long and three inches inside diameter, the cylindricalbody of the chimney is four and one half inches long and one and onehalf inches outside diameter. These dimensions may be as above indicatedvaried within the spirit of my invention. Two insulating Jackets l8 andll of tubing, made out of glass or any other heat resisting material areplaced endwise closely over the body of the chimney, the lower jacketbeing supported by a collar l8 around the lower end of said body. Thefunction of these jackets is to partially insulate, thermally, thecopper body of the chimney from the gases which may bevin the regionbetween the insulating Jackets and the pump casing. Near the lower endof the constricted body of the chimney, a horizontal circular disk 20(see Fig. 3) of thin metal is removably held between the two jackets.This disk is of such diameter that its outside edge is everywhere withina half, of a millimeter (more or less) from the pump casing wall. Thusthe disk performs the function of a baille immediately above thevaporizer and is used to prevent any oil vapor which may be found in thespace below it from rising and interfering with the free movement ofgases which are driven down from above and which are drawn out of theside outlet 2| by the pump F to be hereinafter described. Condensed oilfrom above seals the outer edge of the disk 20 to the casing wall A andat the same time flows downwardly past the disk. A small drain tube 22made out of copper or other suitable material is supplied in the lowerpart of the chimney. Its lower end dips into and is sealed in the oil inthe reservoir and its upper end connects through the wall of the chimneywith the space in the casing A immediately above the lower end portionof the chimney and the baiile disk 20.

The deflector E is mounted rigidly upon the upper end of a shaft 25 bybolt connection 26 and said shaft extends downwardly and rests upon thebottom plate I!) of the chimney and holds the deflector spaced a shortdistance above the chimney in such a position that proper clearance isobtained. Suitable spacing pins 21 radiating outwardly serve to hold theshaft coaxial within the chimney. The lower surface 28 of the deflectorE commencing at the junction of shaft 25 outwardly to its margin isconcave so as to evenly and continuously deflect vapor passing upwardlyfrom the vaporizer through the chimney outwardly and downwardly into thespace between the outer wall of the jacket and the inner wall of thecasing. Both the shaft 25 and deflector are made out of a good heatconductor such as copper or other suitable material, and thereby serveto conduct heat readily from the floor of the vaporizer to the deflectorand prevent the oil vapor from condensing on the inside of thedeflector.

The baflle system F as shown comprises three parts but may be extendedto include as many parts as desired, their function being to prevent oilvapor from ascending into the vacuum chamber B. Each of the three parts(as shown) consists of a portion of a conical surface. The central part30 is made out of one solid piece with.

the outside surface cylindrical in shape to fit snugly into the pumpcasing and at the same time permit easy withdrawal and with its insidesurface 8l frustro conical and pointing downwardly.

The lower and upper members are thin conical shells 32 and, 33, pointingupwardly and held in series spaced relation on the median member 30' bysets of pins 34 and 35. The baflie assembly is supported in the, propervertical position above the deflector E by means of the vertical pin 36,secured rigidly to the lower member'32 and extending downwardly with itslower end resting on theupper end of the shaft 25. The angle of theconical surface of each baflie member of the baflle assembly should becomparatively small for the most eflicient results.

When desired the bafile disk 20 may be provided with a pair ofdeflecting surfaces 31 (see Fig. 3) slanting downwardly from a pointuppermost diametrically opposite the forepump outlet 2|,circumferentially around the insulating jacket I1 and gradually merginginto the plane of the disk for more effectively streamlining anddirecting down draft into said outlet. According to the results desiredthe number of baflie elements may be increased or decreased or removedentirely.

My improved diflusion pump is shown in Fig. 1 connected with a vacuumchamber and forepump as when in operation. In the drawing the upper endof the diffusion pump casing A is connected by sealing to the wall ofthe vacuum chamber B. The outlet duct 2! from the difiusion pump isconnected through a three-way valve 40 with the fore pump F. A branchpassage 4| from said valve to the vacuum chamber B is arranged so thatby turning the valve into one position the forepump acts to exhaust airdirectly from said vacuum chamber for preliminary evacuation. By turningthe valve into another position the forepump acts through my improveddiffusion pump to quickly produce a higher vacuum, and by turning thevalve into a third position both connections from the forepump with thevaccum chamber B and diffusion pump are closed.

Operation Assuming that a high vacuum is desired in chamber B, the valve40 is turned into the position shown in Fig. l and the forepumpoperated. Air or gases are thus exhausted from the chamber B downthrough the diffusion pump and exhaust outlet 2|. During this operationoil in the vaporizer is heated by the heater I4 to a vaporizingtemperature. As the oil boils its vapor ascends to the upper regionsthrough the chimney. Since the oil has a comparatively low heat ofvaporization the chimney D, shaft 25 and deflector E, which are goodheat conductors, prevent the oil vapor from condensing on the lowersurface of the deflector E. This vapor is deflected by the graduallycurved lower surface 28- of the deflector E downwardly into thebombarding region between the outside of the chimney and-inside of thecondenser C. The introduction of this gradually and continuously curveddeflector surface increases the pumping speed of the diffusion pumpmaterially. During this operation gases from the vacuum chamber B aredrawn down into the region between the chimney and the condenser in thediffusion pump casing and are exhausted through the outlet 2| by theforepump. During this operation the oil vapor being directed downwardlypropels the gases in said bombarding region which it bombards downwardlytowards the lower baffle 20. From the lower portion of said bombardingregion immediately above this baflle the gas is drawn off by theforepump through the outlet 2|. The oil apor condenses for the mostpart, on the inner wall of the condenser and in liquid form returns tothe oil vaporizer or reservoir through drain tube 22. The action of thebombarding gases which are exhausted from the vacuum chamber by vapor toproduce a pumping effect is well known and hence a detailed descriptionof the process occurring in said bombarding region is omitted. Sufficeit to say that directing the current of oil vapor into and along withthe current of gases received from the vacuum chamber reduces resistanceto the onward movement of the gases and vapor to a minimum and thusreduces pressure of gases in the 'vacuum chamber effectively in'ashorter space of time than heretofore. vapor is prevented from escapingto the vacuum chamber by the bailieasystem F substantially withoutimpeding the progress of the exhaust flow from the vacuum chamber. Formany purposes the presence of oil or other vapor or even minute tracesthereof from the diffusion pump in the vacuum chamber is highlyundesirable. The bafiie system of conical surfaces of long altitudecharacteristic serves to retard upward passage ofvapor while offering aminimum obstruction to the downward movement of the gases which areremoved from the vacuum chamber. This characteristic in effect is toassist the onward movement of the gases and that characteristic is alsopresent throughout the entire apparatus and produces more effective andtime saving results. In tests which have been made it has beendetermined that at a pressure of .0001 of a millimeter of mercury, themean free path of the gas molecules in the diffusion pump isapproximately 75 C. M. long. Because of this fact it is possible, in thebaffle system disclosed to disregard the collisions which occur betweenindividual molecules of the gas and replace stream line" surfaces withthose surfaces from which molecules bounce in a forward direction, inthe larger number of cases at least. Long na'rrow tapered conicalsurfaces will supply this improvement as has been clearly indicated inpractice.

It will be pointed out that there are two rather critical spacingdimensions which efiect the pumping speed of the pump at any particularpressure. One of these is the distance between the outer edge of thedeflector disc 20 and the inner wall of the pump casing. For a pressureof about 104 m. m. this distance should be about m. m. The other spacingdistance is the vertical clearance between the upper edge of the chimney,D and the lower surface of the deflector E. For this pressure I havefound the optimum distance to be about '7 m. m. All surfaces with whichthe gases and oil vapor come into contact should be well polished andsmooth for the most effective results.

In accordance with the patent statutes, I have described the principlesof operation of my invention, together with the apparatus which I nowconsider to. represent the best embodiment thereof but I desire to haveit understood that the construction shown is only illustrative and thatthe invention can be carried out by other means and applied to usesother than those above set forth within the scope of the followingclaims.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

1. In apparatus for exhausting a closed receptacle, a main ductconnected with said receptacle, a vaporizer supported in the lower endceptacle, a duct communicating with said receptacle, a vaporizersupportedin the lower portion of said duct, a double-walled chimney ofreduced diameter communicating with said vaporizer and providing betweenit and the duct wall a condensing chamber, the outer wall of saidchimney being formed of insulating material, and the inner wall of saidchimney being formed of heat conducting material, said chimney wallsbeing joined to prevent the passage of gases between them.

3. In apparatus for exhausting a closed receptacle, a duct communicatingwith said receptacle, a vaporizer supported in the lower portion of saidduct, a chimney of reduced diameter communicating with said vaporizer,and an insulating wall surrounding said chimney wall and providingbetween it and the wall of said duct 2.

condensing chamber, said walls being closed to the passage of gasesbetween them.

4. A diffusion pump for exhausting a closed receptacle, comprising: aduct communicating with said receptacle, a vaporizer having a reservoirin the lower portion of said duct, a vaporconducting chimney extendingfrom said vaporizer upwardly in said duct, cooling means extending abouta portion of said duct adjacent said chimney, and insulating meansadjacent said chimney and between said chimney and said cooling means.

5. In a diffusion pump for exhausting a closed receptacle, a ductcommunicating with said receptacle, a vaporizer supported in the lowerportion of said duct, a chimney communicating with said vaporizer andextending upwardly therefrom, and a glass tube within said duct whichencircles said chimney, said tube and chimney being closed to thepassage of gases between them.

6. A diifusion pump for exhausting a closed receptacle comprising: aduct communicating with said receptacle; a vaporizer having a reservoirin the lower portion of said duct; a vaporconducting chimney extendingfrom said vaporizer upwardly in said duct; cooling means extending abouta\ portion of said duct adjacent said chimney; an insulating wallsurrounding said chimney; and an annular disk extending radially fromsaid insulating wall and having an external diameter less than that ofthe inside of said duct; said disk and said insulating wall providingwith the wall of said duct a condensing chamber.

7. A diffusion pump for exhausting a closed receptacle comprising: aduct communicating with said receptacle; a vaporizer having a reservoirin the lower portion of said duct; a vaporconducting chimney extendingfrom said vaporizer upwardly in said duct; cooling means extending abouta portion of said duct adjacent said chimney; an insulating wallsurrounding said the wall of said duct an annular passage for thewithdrawal of condensate from said chamber.

8. 4i difl'usion pump for exhausting a closed receptacle comprising: aduct communicating with said receptacle; a vaporizer having a reservoirin the lower portion of said duct; a vaporconducting chimney extendingfrom said vaporizer upwardly in said duct; cooling means extending abouta portion of said duct adjacent said chimney; an insulating wallsurrounding said chimney; an annular disk extending radially from saidinsulating wall and having an external diameter less than that of theinside of said duct, said disk and said insulating wall providing withthe wall of said duct 9. condensing chamber; and a deflector supportedon said disk for guiding the draft within said cooling chamber.

HIRAM W. EDWARDS.

